Seated workers in home, office and industrial environments often experience back pain and other physiological difficulties as a result of ergonomic deficiencies of the various chair designs on the market. Often these difficulties are a result of the absence of appropriate support for the chair user's feet. Due to the problems of discomfort and injury associated with seated work, ergonomic research has been undertaken to identify the causes of these problems and to recommend solutions. In particular, this research has been undertaken to study existing chair designs as well as to develop new chair designs. From this work, six principal chair designs have been identified. Scientific theories have been developed to support claims of ergonomic deficiencies of particular chair designs, as well as to support claims of ergonomic solutions provided by particular chair designs. The chairs embodying these theories are in wide use today.
One such chair design, commonly known as "bench" seating, provides a horizontal seat pan with or without a vertically upright backrest. Users of this chair must exert their back muscles to sit upright while keeping their feet flat on the floor, with their lower legs at a ninety-degree angle to their upper legs. This bench seating or upright posture does allow the sitter to achieve a natural inward curvature of their lower spine without any supportive assistance from the backrest. However, the bench seating posture has been shown by ergonomic researchers to be quite unnatural, as people invariably slouch rearwardly or forwardly into a position in which their back assumed a convex curvature. This tendency to slouch while in bench seating has been shown to occur whether a backrest was provided or not. This is because the bench or upright sitting posture requires a conscious effort to exert the back muscles to hold this position. Recent ergonomic testing has shown that the average person can only maintain this upright bench seating posture for about 31/2 minutes.
Because people invariably succumb to forward or rearward slouching, a vertical backrest is typically added to bench style chairs to help the sitter maintain straightness of the back and minimize the extent of the slouch. However, to make supportive contact with the backrest, sitters on a bench style chair have to lean back slightly from the hip/spine juncture. Adjusting the posture of the upper body by leaning back causes the upper back of the sitter to contact the backrest. This, in turn, creates a gap between the sitter's lower back and the lower portion of the backrest. This rearward slouching, when viewed from the side, appears like the letter C, with an outward bowing, or curvature of the spine. The supportive contact points in this posture are the upper back on the backrest, and the butt on the seat pan. The poor posture resulting from slouching rearward to utilize the backrest for support actually increases the strain on the back and causes even greater outward curving of the lower back. This pathological condition is recognized as a cause of many sitting related back injuries.
Kyphosis is the medical term used to describe the previously mentioned pathological condition of the lumbar area of the spine being curved outward. Kyphosis results in a misalignment of the vertebrae that causes uneven pressure on the vertebral discs. Over time this continuous uneven pressure can rupture or wear down the forward edge of the disc. This is due to the concentration of pressure on only a small local area of the disc. The pressure on the forward edge of the disc is caused by the gravitational pull on the upper body of the seated person. As the seated person assumes a slouched, or kyphotic position, the gravitational forces (upper body weight) are leveraged onto this small frontal disc area. This leveraged pressure on the small frontal disc area is extreme and over time can cause misalignment of the vertebrae. Vertebral misalignment causes pinching of the spinal nerves, which results in extreme back pain that disables the worker.
When standing, the vertebral discs of the human spine are set in a curve that acts to evenly distribute the weight of the upper body throughout the entire surface of the discs so that the entire disc area bears the load. This natural inward curvature of the lower back is referred to as "lordosis". When a person is walking, bending, or lifting, the forces on the discs do concentrate on small specific areas of the discs, but only for brief time periods because the person's motion quickly alternates the pressure from one part of the disc to another. However, when a person is seated, there is little or no muscle movement and therefore no alternation of muscle tension. This static tension registers as pain to the person's nervous system that triggers an instinctual relief mechanism whereby a shift of body position redistributes muscle tension and disc pressure to other locations. This instinctual body shifting is scientifically termed "unconscious mechanataxes". Commonly it is known as squirming. This squirming constantly shifts stressed pressure points and fatigued muscle areas from one surface location or area to another, by alternation. Accordingly, alternation through squirming may be viewed as a natural defense mechanism that acts to relieve static stress.
A bench seated person who is attempting to maintain equalized disc pressure in the lower back through conscious use of back muscles to maintain lordosis (natural inward curving of the spine when standing) invariably fatigues these muscles. In order to relieve this muscle stress, the sitter slouches in his or her chair thus shifting the stress away form one set of back muscles and the evenly pressured discs over to a different set of muscles and the frontal portions of the vertebral discs. This alternation of posture may relieve some muscle discomfort for a while, but at the expense of causing serious injury to the discs. A lumbar support positioned at the lower portion of the backrest offers no improvement towards achieving lordosis because there is nothing forcing the lumbar support into the lower back, as bench posture does not force the user's lower back into the back cushion. Despite these known problems associated with bench seating, it remains the most widely used chair posture in the world.
A second chair design, often called a "Grandjean" chair provides a horizontal seat pan and a backrest which tilts rearward in relation to the horizontal seat pan at an angle between 104 degrees and 113 degrees. This chair was conceived from the observation of rearward slouching of people on their bench style chairs. Accordingly, the reclined backrest was designed to provide support for the sitter's back in the reclined position while the sitter is on a bench chair seat pan. Though the backrest reclined, the load bearing contact points for the sitters were, as with the bench chair, upper back on backrest cushions and butt on seat pan. With the sitter's lower legs perpendicular to the upper legs and feet flat on the floor as recommended, support for the lower back was still negligible. The gap occurring between the lower back and the lower back cushion on the bench chair design was reduced somewhat by the Grandjean reclined backrest but there was still no significant force on the lower back to prevent kyphosis. Because the Grandjean chair encourages rearward slouching, no conscious exertion of back muscles is required, as sitting up straight is not the posture accommodated by this chair design. So, one problem associated with bench seating, muscle fatigue, was solved by Grandjean by encouraging the second problem occurring with the bench chair, slouching.
A third chair design often called a "Mandal" chair provides a seat pan having a forward, downward tilt with an upright back cushion. A. C. Mandal theorized a chair with a forward tilting seat pan incorporated into the overall chair design. He also observed that this tilted seat pan rotated the sitter's pelvis forward causing lordosis of the spine similar to the natural lordosis occurring with the pelvic alignment of a person who is standing. Lordosis is the inward curving of the lower spine that aligns the lumbar discs for even load bearing throughout the entire disc surface. When the method of pelvic rotation is applied to a sitting posture, the benefits are the same as with standing. Neither the Mandal sitting posture nor the standing posture require conscious exertion of back muscles, because the back muscles, in lordosis, are evenly tensed. Neither the standing posture nor the pelvic-rotated sitting posture of Mandal require outside support to maintain the posture. For this reason, the backrest of a Mandal chair is superfluous. However, as the Mandal chair promotes lordosis, chairs of this design provide very secure and comfortable support for the upper body.
The main drawback of the Mandal sitting posture is the tendency for the sitter to slide forward out of the forward sloped seat pan cushion. Chair designers lessened this problem by using rough fabrics to frictionally prevent sliding. This, however, causes skirts and pants to rise up. The most reliable method to prevent forward sliding in these chairs is for the sitter to brace his feet on the floor, or an appropriate foot support to prevent foot sliding. Effective bracing in a Mandal chair is accomplished when the user's lower leg is set from a slightly acute angle to a perpendicular angle to the user's upper leg. This bracing results in statically tensed leg muscles, which may fatigue quickly. Additionally, if the sitter was elevated too high off the floor without an appropriate foot support, there would be no bracing from the feet.
A fourth chair design, often called a "Balans" or "backless" chair provides a seat pan having a forward tilt and a knee rest having a rearward tilt. This chair neither has a backrest nor does it need one as the forward tilting seat pan, like the Mandal, causes a forward pelvic rotation in the sitter. And, as previously mentioned, this forward pelvic rotation positions the upper torso of the user very securely and comfortably. Invented as a solution for the Mandal sliding problem, this chair with its rearward tilting knee cushion set out in front of the forward tilting seat pan, braces against the sitter's knees thus preventing forward sliding without the sitter having to experience muscle fatigue in their legs. This chair design solved the leg muscle fatigue problem of the Mandal chair, but created a new problem unique to its design. The pressure from the knee cushion caused localized pressure point pain on the knees after about twenty minutes of continuous sitting. Larson U.S. Pat. No. 5,330,254 solved this problem by having the front and rear cushions rotate freely thus shifting the knee pressure points constantly up and down along the shins throughout the workday.
A fifth chair design, often called a "center-tilt", was popular in the 1930s and has made a comeback in today's market. Since the seat pan along with the back cushion both rotate backwards together, the gravitational forces (upper body weight) are distributed evenly along the whole longitudinal length of the sitter's spine as it is pushed by gravity against the chair backrest. This results in more even pressures to the upper and lower back areas. And, as the upper body is pushed by gravity against the chair backrest, the chair user will benefit from the lumbar support provided by the chair backrest. In this way, the center-tilt chair eliminates the lower back gap occurring with a bench chair or Grandjean chair. The center-tilt chairs available today also rock back and forth which provide an additional advantage of relocating or shifting the load among the lumbar discs.
There is, however, a significant problem associated with center-tilt chairs. Because the whole body of the sitter rotates backwards, the knees of the sitter rise in relation to the floor as the body rotates backwards. The rising knees lift the sitter's feet off of the floor, leaving the feet and legs dangling in mid air. There is agreement among ergonomic researchers that unsupported legs constitute a very undesirable chair sitting condition due to the deleterious pulling effects that dangling legs have on the sitter's lower back and upper legs. Also, the sitter's lower legs in this chair hang suspended at an angle less than 90 degrees to the upper legs. This condition restricts blood circulation in the legs and causes muscle cramping. Blood circulation is also restricted due to the sharp angle of the seat pan pressing into the back of the legs just above the knee.
Many of the problems associated with the resultant undesirable lifting of the legs caused by leaning back in a center-tilt chair were solved by the knee-tilt chair. The knee-tilt chair, the sixth chair design, relies on a tilting mechanism that is hinged at the front of the seat pan to allow the user to lean back in the chair without causing their legs to rise. I the height of the seat pan is correctly set for a given chair user, the knee tilt chair does provide a sitting posture that allows gravity to push the user into the seat backrest so that slouching is less likely and that a lumbar support, if included in the chair back rest, will provide proper lumbar support.
Like the center-tilt chair, the knee-tilt chair may be undesirable to many workers, as both chairs promote a reclined seating position.
As previously discussed above, all of the six principle chair designs have deficiencies that are specific to their respective design.
A first deficiency shared by all of the chair designs is derived from the fact that most chairs are designed for a single sitting posture and thus do not offer the chair user the ability to vary the sitting position a sufficient amount for performing different tasks. As different tasks in the workplace require different postures for maximizing both comfort and efficiency, a chair that limits the user to a specific posture may require the user to perform a given task in an inappropriate posture. Furthermore, the chair user is often limited to a small amount of adjustment within the specific posture that the chair allows. In this way, many chairs that are claimed to provide healthful sitting restrict the user to limited movement. Restricting movement while sitting often leads to the chair users incurring static stress injuries from the inability to reposition their body as they typically would, were they offered more postural choices. These deficiencies of adjustability are a reality of current chairs, despite the many adjustment provisions that are incorporated into each of these chair designs.
The theories supporting all of the above ergonomic chairs also give little attention or respect to how the feet are supported in each respective chair design. Nevertheless, how the feet are supported in each of these chairs is a determining factor for the effectiveness of each design. All of the previously mentioned chair designs, with the exception of the Balans, have a recommended position where the upper and lower legs are roughly at a 90 degree angle and the feet are placed flat on the floor. The Mandal sitting position also often requires a slight reduction of the angle created by the upper and lower legs to less than 90 degrees. This 90 degree leg and foot position is currently recommended as it has been recognized as necessary to prevent the legs and feet from pulling the user out of the sitting position. Were the legs pulling the user out of the sitting position, there would be excessive back strain to overcome the pulling forces.
Typically a seat pan height adjustment is required for the user to place their legs in the currently recommended position where the feet are placed flat on the floor and the upper and lower legs form an angle of approximately 90 degrees. This height is typically defined as the "popliteal" height. Most work chairs offer height adjustment to accommodate the placement of the seat pan at a desired level to accommodate their leg length. However, a problem encountered with ergonomic seating in the work place is that certain chair users will fall outside of the height adjustment range of their chair, and thus, will not be able to set the seat to a level that is appropriate for their physiological requirements.
People of shorter stature often use chairs that will not vertically adjust to a low enough seat pan height. Unable to set the seat pan at an appropriate level for their required seating position, the users of these chairs are susceptible to both discomfort and injury. With their chairs not adjustable to a low enough height for their stature, these workers are typically unable to position their feet on the floor and as a result may dangle their legs from the chair. Because of the pulling effect of the weight of the legs and feet, these chair users will typically be pulled forward in their chair and caused to sit in a forward slouched position. Unable to use the floor for supportive body bracing, muscle driven lordosis is impossible leaving the user with the typical back rounding condition known as kyphosis.
Workers attempting to position themselves at a high desk or table face a similar problem. In this situation, the desk or table height is not appropriate for upper body comfort until the chair seat is adjusted to the recommended height for their elevated tasking requirements. A common situation encountered in the work place, that exemplifies this problem, is the placement of keyboards of desktop computers and notebook computers on the top of a desk or worktable. Commonly, these keyboards are often at a position that is uncomfortably high for many workers. Workers in this situation are often unable to achieve a comfortable sitting position for the entire body. When the chair seat is adjusted to the recommended level for proper back and leg support, it sets the worker too low for the recommended arm and shoulder comfort. This situation causes the arms to be in a position where the forearms and wrists are susceptible to injury. Shoulder discomfort and injury may also result form this position.
Many workers faced with the use of high work desks and worktables often elevate their chair so as to emphasize proper elbow, arm and hand positions at the expense of foot support. This new seat height that is too high for their feet and legs might cause the user to dangle their legs from the chair causing excessive strain on the remainder of the body to support the legs in this position. By compromising the proper placement of their legs and feet, these workers are typically unable to maintain any supportive contact of their back to the chair backrest and thus tend to slouch in their chair.
The typical solution that is suggested to workers of short stature to improve their posture is to use a footrest to raise their feet above the floor. Once the feet are raised, the user can more easily sit in a proper ergonomic sitting posture that allows full body comfort. The same solution is typically suggested to workers who work at an elevated work height such as a high table and need to raise their seat pan height to accommodate the needs of their upper body. The use of footrests, as is typically suggested by ergonomists in these instances, is for the sole purpose of essentially raising the floor up to the height of the feet so that the feet are supported in the position required by various sitting positions.
"Ergonomic" is typically defined as the efficient fitting of the human being with those inanimate things, especially machines, with which he or she must efficiently interact. "Footrest" is defined as a support for the feet. There are currently six principle footrest designs available for seated people. Each of these principle footrest designs will be described below. The ergonomic functioning of each design will also be evaluated.
One such footrest design consists of a horizontal ring or square attached to a chair at a fixed height between a the chair seat pan and the chair base. In the case of a chair with legs, it would be known as a chair rung. In the case of a chair vertically supported by a non height adjustable center column, the footrest is typically a ring attached to the center column at a fixed height between the seat pan and the chair base. The ring is typically supported outward from the center chair support column with radial spokes.
This footrest design is not considered ergonomic because its location below and inward from the edge of the seat pan requires the user's legs to assume an acute angle to achieve supportive contact. This acute angle (less than 90 degrees) pressures the edge of the seat pan into the underside of the user's thighs which restricts blood circulation and causes leg cramping and nerve tingling. Another non-ergonomic aspect is that even if this chair had seat height adjustability, this footrest design would not adjust correspondingly. In fact, this footrest has no height adjustment at all. And without height adjustability, its already limited usefulness is limited even further to a narrow height range of chair users. Due to the above deficiencies, this footrest is not considered to be ergonomic.
A second footrest design consists of a radial supported ring fastened to the center column of a chair with a sliding sleeve. This ring includes a set screw knob for locking the height adjustment between the seat pan and the chair base. Although this footrest is height adjustable, it shares with the first footrest design the inward location from the seat pan edge and its consequent leg and feet problems. Height adjustment with this footrest design requires the chair user to dismount from the chair and kneel down every time a height adjustment is made. This takes the seated worker away from the work at hand with sufficient inconvenience that the height adjustment capability is seldom utilized. These deficiencies render this footrest design to be considered non-ergonomic.
A third footrest design consists of a horizontal bar or a ring which depends from the underside of a chair seat pan which is mounted on a height adjustable column. When the seat pan is raised or lowered with a gas spring or screw turn device, the footrest correspondingly is raised and lowered. This footrest design approaches ergonomic status but stops short due to the fact that the distance between the footrest and the seat pan is fixed permanently at the factory and therefore has no capability to accommodate the height variances of different chair users. Nor can this footrest be height-set for the popliteal lengths of tall or short people. And, like the first and second footrest designs, this footrest is also located inward of the chair seat's edge, again requiring an acute leg angle under the chair with the resultant feet and leg problems.
A fourth footrest involves the spokes of the chair base doing double duty as both a primary support for the chair and at the same time supporting the feet of the person sitting in the chair. This footrest is in widespread use by many chair users for lack of a better option. One problem with this footrest is that most chairs are swiveled by the chair users throughout the day and this constantly changes the spoke locations in relationship to the swiveling seat pan. This requires the chair user to constantly seek out the new spoke locations rendering feet placement unreliable. Another problem is that chairs with gas spring height adjustability conveniently lift the chair away from the floor-supported spokes. For the chair user who requires elevated seating, this results in the user being prevented from utilizing this footrest. For instance, this footrest would be inaccessible to a tall person who was elevated to drafting table work height, or a short person who was elevated to an appropriate keyboard height. The fixed height of this footrest allows a limited range of chair users whose leg length just happens to be the same distance from the chair base spokes to the seat pan when the chair seat is height adjusted for the user's appropriate work table level. As with the preceding footrest designs, this one is located inward from the seat cushion necessitating an acute leg angle to access it, which causes physiological problems with the legs and feet. There are too many serious deficiencies with this footrest for it to be considered ergonomic.
It should be noted that all four of these footrest designs share one common feature that is considered highly ergonomic. This is that each of the respective footrest locations, inward from the seat cushion, allow the chair user's feet easy access to the floor which allows the user to efficiently maneuver the chair from one location to another. Chair mobility is a valuable ergonomic feature for the modern workplace. This ergonomic feature helps to make the overall chair ergonomic but not the footrest.
A fifth footrest design consists of a floor-mounted platform independent of the chair. This footrest is seldom made to be height adjustable and being independent of the chair, cannot be height adjusted to correspond with height adjustments made to the chair. A single chair user may require differing chair height settings during the day for tasks requiring different table heights such as from keyboard table, to standard table, to drafting table. Another deficiency is that this footrest must be hand carried to different locations where the chair user may perform different tasks. Alternatively, several footrests would have to be purchased for placement in multiple locations. This footrest also does not serve multiple users of a single chair very well when the chair must be height adjusted to accommodate varying user heights, as this footrest does not respond to the height adjustments made to the chair.
Although not typically considered a footrest, the floor is the sixth footrest design. The floor is the universal footrest as it supports the feet directly in a broad range of circumstances such as standing, stooping, running, and sitting. For a chair user, the floor is oftentimes very effective at supporting the feet. It is widely used and provides infinite range for extending the chair user's legs, as was also true of the fifth chair design, the independent platform, whose distal relationship with the chair is floor dependant. The primary limitation of this footrest is the inability of this footrest to be height adjusted to correspond to the user's leg length. The most important requirement for using this footrest is that the seated person's feet are able to reach the floor. If the chair is adjusted too high the user's legs may dangle as the floor does not rise with the chair seat. There are many circumstances where the user is seated too high from the floor for supportive contact with the floor to be possible. Accordingly, this footrest is not considered to be ergonomic due to it's zero height adjustment capability.
The above described footrests have typically been suggested to chair users so that they may assume one of the previously mentioned chair sitting positions that their chair design allows. In most instances, the footrests currently available accomplish the goal of providing chair users a means to elevate their feet to a desired height, albeit with numerous ergonomic and functional deficiencies.
Additionally, all of the above described footrests have not been suggested as a way to improve on the chair sitting positions provided by existing chair designs, despite the deficiencies of each of these designs. For example, someone using a footrest to achieve what is currently recommended as a proper leg and foot position for the bench or Grandjean seating position, will still struggle to obtain adequate backrest support while sitting in the bench or Grandjean position. In the case of the foot ring type footrest, the user is restricted to a foot position that actually inhibits the user in assuming any of the previously mentioned chair sitting positions.
For these reasons, there is a need for an ergonomic footrest mechanism for a work chair that accomplishes the goals of existing footrest mechanisms in allowing chair users of differing body builds (such as those of short stature) and workers using a high desk to position their chair seat at an appropriate height for their upper body while providing support for their feet at this chair height. There is a need that this ergonomic footrest addresses the previously mentioned deficiencies of existing footrest mechanisms by being readily accessible at all of the different locations where the user performs work tasks. There is a need that the ergonomic footrest mechanism allows independent adjustability of the position of the footrest in relation to the chair seat pan to accommodate differing body builds that may use the chair. This independent adjustability is also necessary to accommodate the differing postures that may be required by each user to accomplish different tasks. There is also a need for an ergonomic footrest mechanism that can be efficiently retracted out of the way for easy foot access to the floor.
Most importantly, there is a need for an ergonomic footrest mechanism that does not merely allow the user to properly assume a currently recommended sitting posture that is provided by the chair design of their choice, but addresses the deficiencies of the existing recommended sitting postures provided by current chair designs. In particular, there is a need for an ergonomic footrest mechanism that allows chair users of all body builds sitting in the postures provided by any of the following chair designs: bench, Grandjean, Mandal, Balans, and center-tilt, and knee-tilt; to first maximize the benefits of each of these designs; and second to improve the postural functioning of each of these chair designs. For example, users of the bench and Grandjean chairs have a need for an ergonomic footrest that will provide them an enhanced ability to maintain better supportive contact of the lumbar region of their back on the chair's backrest. This supportive contact is essential for maximizing lower back support and thus minimizing discomfort and injury. There is further need for an ergonomic footrest mechanism that improves on each of these current designs by also allowing ease of positional adjustability or variance that each of these current designs restrict. This positional adjustability is necessary for the benefit of the chair user in assuming a correct posture for differing work tasks, as well as to allow users to reposition their body to minimize static stress. There is a need for an ergonomic footrest mechanism of this type which can be attached to existing work chair designs including elevated stools as well as non-elevated office type work chairs. There is also a need that the ergonomic footrest mechanism be both attractive in appearance and inexpensive to manufacture.